The present invention relates to a driving torque (or driving force) distribution control system for a four wheel driving vehicle, and more specifically to a fail-safe control system which can maintain the stability of the vehicle even when a driving force distribution control system fails.
Japanese Patent Provisional Publication No. 62-46716 discloses one conventional example. A control system of this example is designed to change the four wheel drive system to the 2WD state when an abnormal condition is detected. In the two wheel drive state, all the driving torque is applied only to the rear wheels, and the rear wheel cornering force is decreased. Therefore, an abrupt change from the 4WD state to the rear two wheel drive state is liable to cause a spin of the vehicle especially when the change is made during a cornering operation near the critical lateral acceleration. In the vehicle in which all the driving torque is transmitted to the front wheels in the 2WD state, an abrupt change from the 4WD state to the two wheel drive state is liable to cause a drift-out of the vehicle due to decrease in the front wheel cornering force.